The WIEN BRIDGE OSCILLATOR (figure shown below) largely relies on a pair of LR resonant networks for shifting the output signal by exactly 180 degree and one particular frequency. Then this phase-shifted signal is fed back to the non-inverting (1) input of the op amp. It is amplified and add force to the output signal. A strong sinusoidal waveform is generated. In Wien network, the two capacitors and both od the resistors are equal in value. The signal frequency is f = 1/2πRC.
The benfit of using the amplifier is that it is easily made tunable by making use of a dual ganged variable resistor for the 2 resistors in the Wien network.
The gain of the op amp is required to be kept closer to 3. Oscillations will be vanished away if gain is under three, and is greater than three the op amp will become saturated. This gain is set by the variable feedback resistor and the resistance of the filament lamp. The value of resistance faced in lamp depends on the value of average current flowing through it, which depends on the amplitude of the signal. When there is high gain and high current the filament heats up very soon and resistance in it also increases. The gain is decreased by this increase of resistance. If gain falls down, the current also falls and the filament cools down. The resistance of the lamp decreases and gain is reduced. This is the fine illustration of –ve feedback heading towards stability. The point at which the gain is stabilized is set by the variable resistor. This is adjusted until a very strong and permanent signal is produced at the output terminal. After this the amplifier has the required gain, which is close to 3.
Things to do
· Try bread boarding these oscillators.
· View their output signals with an oscilloscope, or
· measure them using a test meter with a frequency measuring facility.
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